Stent With Improved Flexibility and Method for Making Same

ABSTRACT

A stent and a method for manufacturing a stent are provided. The stent includes a first ring having a plurality of peaks and a plurality of valleys, a second ring having a plurality of peaks and a plurality of valleys, and a connector that connects one of the peaks of the first ring to one of the valleys of the second ring. The connected peak of the first ring includes a deformed portion that extends towards the connected valley of the second ring. The method includes forming a first ring having a plurality of peaks and a plurality of valleys, forming a second ring having a plurality of peaks and a plurality of valleys, deforming a portion of at least one of the peaks of the first ring, and connecting the deformed portion of the peak of the first ring to one of the valleys of the second ring.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a stent with improvedflexibility. More specifically, the present invention relates to awelded stent having increased flexibility at the welded connections.

2. Description of Related Art

A stent is a prosthesis that is inserted into a body lumen and used, forexample, for treating stenoses, strictures, and/or aneurysms therein. Inthe event of a stenosed vessel, a stent may be used to prop open thevessel after an angioplasty procedure. Once opened, the stent forms tothe inner wall of the vessel, remains in place, and may help preventrestenosis. Additionally, in the event of an aneurysm or weakened vesselwall, stents may be used to provide support to, and reinforce the vesselwall.

To perform such functions, stents in the past have included manydifferent structures. For example, previously disclosed stents includecoiled stainless steel springs, helical wound springs, and generallyserpentine configurations with continuous waves of generally sinusoidalcharacter. Some of these stents self deploy when placed in the vessel,whereby stent expansion is primarily achieved by removing a restraintmechanism holding the stent in a constricted state. Other types ofstents rely on alternate means to deploy, for example, use of a ballooncatheter system, whereby balloon dilation expands and deploys the stent.

One of the major complications associated with using stents has beenthrombosis. This complication is caused by clotting in the vicinity ofthe stent and is associated with high morbidity and mortality. It hasbeen shown that the better the stent apposition against the vessel walland the larger the lumen, the less likely that this complication willoccur. A further complication is restenosis, which is caused by tissueproliferation around the angioplasty site. To minimize the potential forrestenosis, the stent should cover the lesion and not leave anysignificant gaps in which restenosis may occur. The stent should alsoadhere to the inner wall of the vessel as much as possible.

Accordingly, when a stent deploys in a restricted vessel, adequateradial strength is required to overcome the strictures and ensureapposition of the stent to the vessel wall. Radial strength is a forceproduced by the stent acting at all points on the vessel wall in anoutwardly direction perpendicular to the vessel wall. Stents aredesigned with circumferential rings to provide most of the radialstrength needed to overcome radial forces pushing inwardly against thestent as the stent expands.

Many stents also include longitudinal links that primarily act to attachlongitudinally adjacent circumferential rings, but also add radialstrength and stent stability. Once the stent is fully deployed, inaddition to providing adequate radial strength, the stent must provideadequate vessel wall coverage, hereinafter referred to as scaffoldingaffect. Scaffolding affect is defined as the amount of area of thevessel wall covered by the stent, once the stent is fully deployed. Thecircumferential rings and longitudinal links connecting thecircumferential rings have traditionally provided the needed scaffoldingaffect. Other stents include welded connections between longitudinallyadjacent circumferential rings.

Further, to meet the demands of adequate radial strength and scaffoldingaffect, conventional stents have been designed with circumferentialrings manufactured with adequate ring width, which were thencontinuously connected at each peak and valley or trough by longitudinallinks. However, such conventional stents suffer from predilation stentlongitudinal rigidity. Predilation or crimped stent longitudinalrigidity is a resistance to movement and decreased flexibility of thestent along the stent's longitudinal axis. Accordingly, predilationlongitudinal stent rigidity makes it much harder and oftentimes evenimpossible to thread the stent through long tortuous vessels and pastconstrictions and lesions.

Past attempts have been made to overcome predilation stent longitudinalrigidity. Such attempts have included designs with decreased ring width,often referred to as decreased wire gauge, which resulted in increasedlongitudinal flexibility but decreased radial strength. Theseconventional designs have resulted in inadequate stent apposition and/orinadequate vessel wall support. Additionally, past attempts to increaselongitudinal flexibility have included designs where longitudinal linksare not attached to each peak and valley of the circumferential ring.Thus, only some of the peaks and valleys of adjacent circumferentialrings are connected by longitudinal links. This increases longitudinalflexibility but decreases the scaffolding affect of the stent. Thedecreased scaffolding affect creates areas where the vessel wall is notadequately covered by the stent, which may lead to thrombosis and/orrestenosis.

Additionally, in order to meet the requirements of drug eluting stents,conventional stent substrates have been designed with circumferentialelements manufactured with adequate ring/strut/apex width, which werethen continuously connected at each peak and valley by longitudinallinks. However, such conventional stents may suffer from abrasion ordamage due to adjacent apexes (i.e., peaks and valleys) interactingduring crimping and tracking, which may be caused by the close proximityof adjacent apexes coming into contact with one another due to links orweld not providing adequate clearance. This interaction may causeabrasion or damage during the coating of the stent with a drug and/orpolymer or during tracking of the stent through the anatomy.

Accordingly, there arises the need for a stent, which provides adequateradial strength, scaffolding affect, with increased apex spacing andlongitudinal flexibility. It is among the objects of the presentinvention to provide a stent that overcomes the foregoing shortcomingsand meets the needs discussed above.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a stent having improvedlongitudinal flexibility and minimal apex to apex (i.e., peak to valley)interaction between adjacent rings.

In an embodiment of the present invention, a stent is provided. Thestent includes a first ring having a plurality of peaks and a pluralityof valleys, a second ring having a plurality of peaks and a plurality ofvalleys, and a connector connecting one of the peaks of the first ringto one of the valleys of the second ring. The connected peak of thefirst ring includes a deformed portion that extends towards theconnected valley of the second ring.

Another aspect of the present invention provides a method formanufacturing a stent with improved longitudinal flexibility andincreased apex to apex spacing between adjacent rings.

In an embodiment of the present invention, a method for manufacturing astent is provided. The method includes forming a first ring having aplurality of peaks and a plurality of valleys, forming a second ringhaving a plurality of peaks and a plurality of valleys, deforming aportion of at least one of the peaks of the first ring, and connectingthe deformed portion to one of the valleys of the second ring.

In another embodiment of the present invention, a method formanufacturing a stent is provided. The method includes forming a firstring having a plurality of peaks and a plurality of valleys, forming asecond ring having a plurality of peaks and a plurality of valleys,connecting one of the peaks of the first ring to one of the valleys ofthe second ring, and deforming a portion of the connected peak of thefirst ring.

The foregoing and other features and advantages of the invention willbecome further apparent from the following detailed description of thepresently preferred embodiments, read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the invention, rather than limiting the scope of theinvention being defined by the appended claims and equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich corresponding reference symbols indicate corresponding parts, andin which:

FIG. 1 illustrates a stent in accordance with an embodiment of theinvention;

FIG. 2 illustrates a detailed view of a connection between two adjacentrings of a conventional welded stent;

FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 2;

FIG. 4 illustrates a detailed view of a connection between two adjacentrings of a stent according to an embodiment of the invention;

FIG. 5 is a cross-sectional view taken along line 5-5 in FIG. 4;

FIG. 6 is a perspective view of a distal end of an embodiment of a toolused to manufacture the stent of FIG. 1;

FIG. 7 is a side view of the tool of FIG. 6 just before it is applied toone of the rings of FIG. 4 in accordance with an embodiment of theinvention;

FIG. 8 is a side view of the tool of FIG. 6 just after it has beenapplied to the ring of FIG. 7;

FIG. 9 is a side view of a portion of an apparatus used to manufacturingthe stent of FIG. 1 according to another embodiment of the invention;and

FIG. 10 is a perspective view of a distal end of a tool of the apparatusof FIG. 9.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The foregoing and other features and advantages of the invention will beapparent from the following, more detailed description of the preferredembodiment of the invention, as illustrated with reference to theFigures. While specific embodiments are discussed in detail, it shouldbe understood that this is done for illustrative purposes only. A personskilled in the art will recognize that other embodiments can be usedwithout departing from the spirit and scope of the invention.

FIG. 1 illustrates a stent 10 according to an embodiment of theinvention. As illustrated, the stent 10 includes a plurality ofcircumferential rings 12 that are each in the shape of a sinusoid. Eachring 12 includes a plurality of peaks 14 and a plurality of valleys 15that are connected to each other by a plurality of segments 16. Aproximal end 18 of the sinusoid has been arbitrarily labeled “peak” anda distal end 20 of the sinusoid has been arbitrarily labeled “valley.”It would be understood by one of ordinary skill in the art that thepeaks 14 and valleys 15 have been labeled for illustrative purposes andease of understanding and that the terms may be switched.

Each ring 12 may be formed from a single piece of material, such as ametal wire, or each ring/element 12 may be cut from a metal tube. Forrings 12 that are formed from a single wire, the ends of the wire may bewelded together so as to form a continuous ring. The material used tofabricate the rings 12 can be made of an inert, biocompatible materialwith high corrosion resistance that can be plastically deformed atlow-moderate stress levels such as tantalum, or moderate to high stresslevels such as L605, MP35N, or any other high work hardening ratematerial. Other acceptable materials include, but are not limited to,nickel titanium, stainless steel, titanium ASTM F63-83 Grade 1, niobium,cobalt-chromium (Co—Cr) alloys, and other cobalt-based alloys. Aself-expanding device can be made by the use of superelastic NiTi, suchas nitinol. As discussed in further detail below, a single ring may beconnected to an adjacent ring with a connector 22, such as a weld 24, soas to form a flexible connection between the rings.

FIGS. 2 and 3 illustrate a conventional connection between adjacentrings 12 a′, 12 b′ of a stent 10′. As illustrated, the rings 12 a′, 12b′ may be connected with a connector 22′. In the illustrated embodiment,the connector 22′ is a weld 24′. During the manufacturing process, theadjacent rings 12 a′, 12 b′ are placed in contact with each other sothat a peak 14 a′ of one ring 12 a′ contacts a valley 15 b′ of anadjacent ring 12 b′. The weld 24′ is then created at the contact pointof the peak 14 a′ and valley 15 b′ so as to form the connector 22′. Theweld 24′ may be created by conventional welding techniques, includingbut not limited to butt welding, resistance welding, and/or laserwelding. As shown in FIG. 2, the resulting weld 24′ has a length of d₁.Such a configuration may provide a connection with limited flexibility,because the peak 14 a′ and valley 15 b′ are abutted against each other,and the length d₁ of the weld 24′ is relatively short. In order toincrease the length of the weld, the peaks 14 a′ and the valley 15 b′ ofthe adjacent rings 12 a′, 12 b′ would have to be spaced further apartbefore the weld 24′ is created, which may lead to inconsistent weldlengths and/or weaker connections.

FIGS. 4 and 5 illustrate a connection between adjacent rings 12 a, 12 baccording to an embodiment of the invention. As illustrated, a peak 14 aof one of the rings 12 a includes a deformed portion 26 a andnon-deformed portions 28 a that are on opposite sides of the deformedportion 26 a. Similarly, a valley 15 b of the adjacent ring 12 bincludes a deformed portion 26 b and non-deformed portions 28 b that areon opposite sides of the deformed portion 26 b. The deformed portions 26a, 26 b may be created by methods discussed in further detail below. Inthe illustrated embodiment, the rings 12 a, 12 b are connected with aconnector 22 in the form of a weld 24.

As shown in greater detail in FIG. 5, the deformed portions 26 a, 26 beach include a recess 30 a, 30 b, respectively, that are recessed fromthe respective non-deformed portions 28 a, 28 b of the peak 14 a and thevalley 15 b, respectively. As discussed in further detail below, as eachrecess 30 a, 30 b is created, at least one extension 32 a, 32 b,respectively, is also created, due to the displacement of the material.In an embodiment, one of the extensions 32 a of the recess 30 a extendsin a direction that is toward the valley 15 b of the adjacent ring 12 b,as shown in FIG. 6, and the other extension 32 a extends in a directionthat is away from the valley 15 b of the adjacent ring 12 b. Althoughtwo extensions 32 a are illustrated, in some embodiments, the recess 30a may only include a single extension that extends towards the valley 15b of the adjacent ring 12 b. The illustrated embodiment is not intendedto be limiting in any way.

As discussed in further detail below, the deformed portions 26 a, 26 bmay be created by work hardening (e.g., cold-working) the material inthe peak 14 a and the valley 15 b, respectively, such that the materialplastically deforms, thereby creating the recesses 30 a, 30 b and theextensions 32 a, 32 b. As a result of work hardening the material, thedeformed portions 26 a, 26 b may have a hardness that is greater thanthe hardness of the non-deformed portions 28 a, 28 b of the peak 14 aand the valley 15 b, respectively.

In an embodiment, the material in the deformed portions 26 a, 26 b havea hardness that is at least about 20%, and preferably between about 20%and about 40%, higher than the hardness of the material in thenon-deformed portions 28 a, 28 b due to the work hardening of thematerial. For example, in an embodiment, the ring 12 a may be made fromannealed stainless steel, or Co—Cr alloy having a Vickers hardness ofabout 220 HV, while the hardness of the material of the deformed portion26 a that has been work-hardened may be about 300 HV, which is anincrease of about 36%.

Of course, the actual amount of increase in hardness of the material inthe deformed portion 26 a will depend on the material, the degree ofdeformation, the working temperature, and the amount and duration ofpressure that is applied to the material. The same considerations applyto the deformation of the valley 15 b of the adjacent ring 12 b, ifapplicable. In some embodiments, only the peak of one ring is deformedand is connected to a non-deformed valley of an adjacent ring. Theillustrated embodiment is not intended to be limiting in any way.

By creating the deformed portions 26 a, 26 b in the peak 14 a and thevalley 15 b, respectively, by work hardening the material, not only arethe extensions 32 a, 32 b created, but the strength of the extensions 32a, 32 b may be increased. This may allow the connection between the peak14 a and the valley 15 b to be more flexible, yet stronger. Increasedflexibility may be achieved by allowing the non-deformed portions 28 a,28 b of the peak 14 a and the valley 15 b in the adjacent rings 12 a, 12b, respectively, to be spaced apart at a greater distance than otherconnected peaks and valleys, such as the peak 14 a′ and valley 15 b′illustrated in FIGS. 2 and 3 and discussed above.

For example, as discussed above, in the conventional welded stent 10′,the weld 24′ has a length of d₁. However, in the embodiment illustratedin FIG. 4, the weld 24 of the stent 10 has a length d₂, which is greaterthan d₁ due to the presence of the extensions 32 a, 32 b of the deformedportions 26 a, 26 b of the peak 14 a, and the valley 15 b, respectively.The longer weld 24 (as compared to the weld 24′ illustrated in FIGS. 2and 3) may improve the flexibility of the connector 22, while the workhardened material in the deformed portions 26 a, 26 b may increase thestrength of the connector 22. In other words, the presence of theextensions 32 a, 32 b within the weld 24 may increase the strength ofthe connection between the adjacent rings 12 a, 12 b, and at the sametime provide a more flexible connection.

The weld 24 may be created by conventional welding techniques, includingbut not limited to butt welding, resistance welding, and/or laserwelding. In addition, it is contemplated that the connector 22 may notbe in the form of a weld per say, and may be created by soldering orbrazing.

In an embodiment, heat may be generated at the peak 14 a and the valley15 b with a laser, so as to cause the material in the peak 14 a and thevalley 15 b to flow together, thereby creating the weld 24. As the weld24 is created, an inert gas, such as argon or helium, may be used toflood the weld area at a sufficient flow rate to prevent oxidation sothat the weld 24 does not become brittle. Of course, other weldingtechniques may be used, and the above-described method should not beconsidered to be limiting in any way.

FIG. 6 shows an embodiment of a tool 40 that may be used to create thedeformed portion 26 a of the peak 14 a of the ring 12 a described above.Of course, the same tool 40 may be used to create the deformed portion26 b of the valley 15 b of the ring 12 b as well. The tool 40 ispreferably fabricated from a material having a greater hardness than thematerial used to form the ring 12 a. In the embodiment illustrated inFIG. 6, the tool 40 includes a punch 42 that has a circularcross-section and a distal end 44 that is flat. Of course, the punch 42may have other cross-sectional shapes, such as ellipsoid, rectangular,etc. The illustrated embodiment of the punch 42 is not intended to belimiting in any way.

A support 46 may be placed inside the ring 12 so that it contacts aninside surface 48 of the peak 14 a, as shown in FIG. 7. The support 46may be a mandrel, or any other structure that is configured to supportthe ring 12 a as the punch 42 is used to create the deformed portion 26a of the peak 14 a. After the ring 12 a has been properly positioned onthe support 46, the punch 42 may engage an outside surface 50 of thepeak 14 a at a location of the peak 14 a where the deformed portion 26 ashould be created. With the peak 14 a positioned between the support 46and the punch 42, suitable pressure may be applied to the punch 42 untilthe desired amount of deformation takes place, as shown in FIG. 8. Thesuitable pressure should be enough pressure to cause the material of thepeak 14 a to flow, yet not too great to cause the material at the peak14 a to fracture. As would be appreciated by one of ordinary skill inthe art, the stress-strain curve of the material used to form the rings12 may be used to select the suitable pressure. After the desired amountof deformation has taken place, the punch 42 may be removed from thepeak 14 a, thereby leaving the deformed portion 26 a behind.

In another embodiment, the deformed portions 26 a, 26 b may be formedsimultaneously by using an apparatus 52 illustrated in FIG. 9. Asillustrated, the apparatus 52 includes a mandrel 54 on which the rings12 of the stent 10 are placed. The apparatus 52 also includes aplurality of tools 40 that are aligned along the mandrel 54 such thatthe tools 40 are axially aligned with the peaks 14 of the rings 12. Inan embodiment, illustrated in greater detail in FIG. 10, the tool 40includes a roller 56 at a distal end thereof. The mandrel 54 may berotated and the tools 40 may be moved towards the mandrel so that eachroller 56 may engage the outer surfaces 46 of the peaks 14. As shown, asingle roller 56 may engage a peak 14 of one ring and a valley 15 of anadjacent ring 12 at the same time. Of course, other arrangements may beused. For example, if deformed portions 26 are to be created in onlycertain peaks 14 and/or valleys 15, rather than all of the peaks 14 andvalley 15, the mandrel 54 and the rollers 56 may be indexed so that therollers 56 only contact the peaks 14 in which the deformed portions 26are to be created. The illustrated embodiment is not intended to belimiting in any way.

In another embodiment, that peak 14 a and the valley 15 b of adjacentrings 12 a, 12 b may first be welded together, as shown in FIG. 2, andthe tool 40 (either with the punch 48 of FIG. 6 or the roller 56 of FIG.10) may then be used to create the deformed portions 26 a, 26 b of thepeak 14 a and the valley 15 b, respectively, in the manner discussedabove. Both methods are contemplated as being within embodiments of thepresent invention.

While the embodiments of the invention disclosed herein are presentlyconsidered to be preferred, various changes and modifications can bemade without departing from the spirit and scope of the invention. Thescope of the invention is indicated in the appended claims, and allchanges that come within the meaning and range of equivalents areintended to be embraced therein.

1. A method for manufacturing a stent, the method comprising: forming afirst ring having a plurality of peaks and a plurality of valleys;forming a second ring having a plurality of peaks and a plurality ofvalleys; deforming a portion of at least one of the peaks of the firstring; and connecting the deformed portion to one of the valleys of thesecond ring.
 2. The method according to claim 1, wherein the first ringis formed from a first continuous wire.
 3. The method according to claim2, wherein the second ring is formed from a second continuous wire. 4.The method according to claim 1, wherein said deforming comprises workhardening the portion.
 5. The method according to claim 4, wherein theVicker hardness value of the deformed portion of the peak increases byat least about 20% during said work hardening.
 6. The method accordingto claim 5, wherein the Vicker hardness value of the deformed portion ofthe peak increases by about 20% to about 40% during said work hardening7. The method according to claim 1, further comprising deforming aportion of at least one of the valleys of the second ring, and whereinsaid connecting comprises connecting the deformed portion of the atleast one peak of the first ring to the deformed portion of the at leastone valley of the second ring.
 8. The method according to claim 7,wherein said deforming the portion of the at least one valley of thesecond ring comprises work hardening the portion of the at least onevalley.
 9. The method according to claim 8, wherein the Vicker hardnessvalue of the deformed portion of the valley of the second ring increasesby at least about 20% during said work hardening.
 10. The methodaccording to claim 9, wherein the Vicker hardness value of the deformedportion of the valley of the second ring increases by about 20% to about40% during said work hardening.
 11. The method according to claim 7,wherein the deformed portion of the peak of the first ring and thedeformed portion of the valley of the second ring contact each otherduring said connecting so as to space apart non-deformed portions of thepeak and non-deformed portions of the valley.
 12. The method accordingto claim 1, wherein said deforming comprises pressing a tool against theportion.
 13. The method according to claim 12, wherein the toolcomprises a punch.
 14. The method according to claim 12, wherein thetool comprises a roller.
 15. The method according to claim 1, whereinsaid connecting comprises welding.
 16. A method for manufacturing astent, the method comprising: forming a first ring having a plurality ofpeaks and a plurality of valleys; forming a second ring having aplurality of peaks and a plurality of valleys; connecting one of thepeaks of the first ring to one of the valleys of the second ring; anddeforming a portion of the connected peak of the first ring.
 17. Themethod according to claim 16, further comprising deforming a portion ofthe connected valley of the second ring.
 18. The method according toclaim 17, wherein said deforming the portion of the connected valley ofthe second ring comprises work hardening.
 19. The method according toclaim 18, wherein the Vicker hardness value of the deformed portion ofthe valley of the second ring increases by at least about 20% duringsaid work hardening.
 20. The method according to claim 19, wherein theVicker hardness value of the deformed portion of the valley of thesecond ring increases by about 20% to about 40% during said workhardening.
 21. The method according to claim 16, wherein said deformingcomprises work hardening.
 22. The method according to claim 21, whereinthe Vicker hardness value of the deformed portion of the peak increasesby at least about 20% during said work hardening.
 23. The methodaccording to claim 22, wherein the Vicker hardness value of the deformedportion of the peak increases by about 20% to about 40% during said workhardening.
 24. The method according to claim 16, wherein said deformingcomprises pressing a tool against the portion.
 25. The method accordingto claim 24, wherein the tool comprises a punch.
 26. The methodaccording to claim 24, wherein the tool comprises a roller.
 27. Themethod according to claim 16, wherein said connecting comprises welding.28. A stent comprising: a first ring having a plurality of peaks and aplurality of valleys; a second ring having a plurality of peaks and aplurality of valleys; and a connector connecting one of the peaks of thefirst ring to one of the valleys of the second ring, the connected peakof the first ring comprising a deformed portion that extends towards theconnected valley of the second ring.
 29. The stent according to claim28, wherein the deformed portion of the peak has a Vickers hardnessvalue that is at least about 20% greater than the Vickers hardness valueof non-deformed portions of the peak.
 30. The stent according to claim29, wherein the deformed portion of the peak has a Vickers hardnessvalue that is about 20% to about 40% greater than the Vickers hardnessvalue of non-deformed portions of the peak.
 31. The stent according toclaim 28, wherein the connected valley of the second ring comprises adeformed portion that extends towards the connected peak of the firstring.
 32. The stent according to claim 31, wherein the deformed portionof the valley of the second ring has a Vickers hardness value that is atleast about 20% greater than the Vickers hardness value of non-deformedportions of the valley of the second ring.
 33. The stent according toclaim 32, wherein the deformed portion of the valley of the second ringhas a Vickers hardness value that is about 20% to about 40% greater thanthe Vickers hardness value of non-deformed portions of the valley of thesecond ring.
 34. The stent according to claim 31, wherein the deformedportion of the peak of the first ring and the deformed portion of thevalley of the second ring contact each other so as to space apart thenon-deformed portions of the peak and the non-deformed portions of thevalley.
 35. The stent according to claim 31, wherein the connectorcomprises a weld that contacts the deformed portion of the peak of thefirst ring and the deformed portion of the valley of the second ring.36. The stent according to claim 35, wherein the connector comprises aweld that contacts the deformed portion.
 37. The stent according toclaim 28, wherein the first ring is formed from a first continuous wire.38. The stent according to claim 37, wherein the second ring is formedfrom a second continuous wire.